This invention relates to the input side rectifier in an ac drive system and more particularly to the inductors used therein.
The two main subsystems of a modern ac drive system are the input side rectifier system and output inverter system. The purpose of the rectifier system is to convert input ac voltage, from the utility source, into an intermediate dc voltage and the purpose of the inverter system is to convert the intermediate dc voltage into a variable frequency and a variable magnitude ac output voltage. The rectifier systems are also used in equipment such as welding, electroplating and uninterruptible power supplies.
The input rectifier system consists of a three-phase diode bridge, either ac or dc side inductor(s) and dc bus capacitors. The three-phase diode bridge converts input ac voltage into dc voltage. The inductor(s) and capacitor(s) serve as a smoothing filter for the intermediate dc voltage. Such a rectifier system, when connected to a sinusoidal voltage utility source, draws non-sinusoidal currents. These harmonic currents are not desirable because of their adverse effects (such as energy losses and malfunction of the sensitive equipment) on the utility network. Therefore, it is of commercial importance to reduce the harmonic currents produced by the rectifier systems.
The magnitudes of the harmonic currents are mainly dependent on the value of the ac or dc side inductors and on the average value of the load current on the dc side. Generally speaking, the level of the harmonic line currents commonly measured in percent total harmonic distortion (%THD) is lower if the value of the inductor is large. But the larger the value of the inductor, the bigger it is in size and the more expensive it is. Also the %THD increases as the load on the rectifier circuit is reduced from full load to partial load. Since ac drives operate at partial load for most of their operating time, it is important to minimize the %THD of a rectifier circuit at partial load.
The present invention reduces the %THD of the line current of a rectifier circuit by incorporating one or more non-linear inductor(s) in the ac or dc side of the rectifier circuits. Specifically, the invention reduces the %THD of the line current at partial loads when compared with rectifier circuits using conventional (linear) inductors. As an additional benefit, the invention also reduces current ripple stress on the filter capacitor at partial loads.
A system that has a rectifier having an input for connection to an ac source and an output; and an inductance-capacitance filter connected to the output of the rectifier where the inductance comprises one or more inductors designed to have an inductance that changes non-linearly over a predetermined range of operating current and harmonic currents drawn from said ac source both when a rated load and a partial load are connected to said rectifier that have a percent total harmonic distortion that is less than the percent total harmonic distortion when said inductance comprises one or more linear inductors.
A system that has a rectifier having an input for connection to an ac source and an output; one or more inductors designed to have an inductance that changes non-linearly over a predetermined range of operating current and harmonic currents drawn from said ac source both when a rated load and a partial load are connected to said rectifier that have a percent total harmonic distortion that is less than the percent total harmonic distortion when said inductance comprises one or more linear inductors connected between the input of the rectifier and the ac source; and a capacitor connected across the output of the rectifier.
A non-linear inductor that has a magnetic material tape wound toroidal core; and an air gap in the core having at least two widths.